In general, LED based lighting applications (also referred to as LED units, comprising one or more light emitting diodes (LEDs)) are powered by an LED driver (e.g. comprising a switched mode power supply such as a Buck or Boost converter) which is supplied from a DC voltage source. In such an arrangement, dimming of the light (in response to a user interface action) is typically realised by adjusting the duty cycle of the LED or LEDs of the application. As such, conventional LED drivers are not suited for being powered by a voltage source which differs from a DC voltage source, such as e.g. provided by an electronic transformer or a standard TRIAC dimmer. When an LED driver is supplied from a varying voltage source, the instantaneous voltage available as input to the LED driver may be momentarily insufficient to power the LED or LEDs of the lighting application. This could result in flicker of the lighting application which could result into a range of effects in an observer, from awkwardness via irritation to nausea.
When such a varying voltage source such as an electronic transformer or a TRIAC dimmer is used to power a normal halogen light, the power received will be averaged out and will not result in flicker, although even with halogen lights, the low output levels are cumbersome and flicker can be seen in many cases.
In case a lighting application is powered from an electronic transformer, it is further required to, in order for the electronic transformer to provide an output voltage for supplying the LED unit, maintain a current as provided by the transformer above a certain level. As such, when the supply current (i.e. the current supplied to the LED driver) is insufficient, the electronic transformer will cease to provide an output voltage. Subsequently, the electronic transformer will, after a certain amount of time, attempt to resume its proper operation. Meanwhile however, the light output of the LED unit could be interrupted, whereas a continuous light output would be desired. In order to ensure proper operation of the electronic transformer, it has been proposed in literature to provide a load in parallel to the LED driver in order to ensure that a minimum supply current is being supplied by the electronic transformer. Maintaining such a current may result in an important dissipation, adversely affecting the efficiency of the lighting application.
Ensuring that a sufficiently high supply current is provided by an electronic transformer powering one or more LED drivers, is rendered even more difficult because of the high power to light conversion of LED based lighting applications, compared to conventional halogen lights. As will be understood, when a 20 W halogen light bulb is replaced by a 5 W LED unit, the power to be supplied by the electronic transformer can be reduced significantly and may result in a supply current insufficient for proper operation of the electronic transformer.
In view of the above, it is an object of the present invention to facilitate the powering of LED based lighting applications by an electronic transformer (optionally preceded by a TRIAC based dimmer), thereby facilitating conventional applications such as halogen lights to retrofit with LED based lighting application.